The invention relates to a process for converting synthesis gas comprising hydrogen and carbon monoxide to hydrocarbons using a zeolite-methanol catalyst system containing a methanol catalyst and a molecular sieve zeolite catalyst.
Processes are known in which a gaseous feedstock is converted to an intermediate product and the intermediate product is converted to a liquid hydrocarbon fuel product simultaneously through the use of catalyst systems comprising mixtures of catalysts. Zeolite-methanol catalyst systems have been used for producing hydrocarbons boiling in the gasoline range from synthesis gas, also referred to as syngas. Such catalyst systems contain a high-temperature methanol catalyst, such as ZnO—Cr2O3, capable of converting the syngas to methanol, and a molecular sieve zeolite catalyst, such as ZSM-5, capable of converting the methanol to hydrocarbons. U.S. Pat. Nos. 4,011,275 and 4,180,516 and 4,331,774 each disclose catalyst systems comprising such combinations in order to obtain highly aromatic hydrocarbon products.
Zeolite catalysts are well-known in the processing of petroleum and in the production of various petrochemicals. ZSM-5, disclosed and claimed in U.S. Pat. No. 3,702,886, is a widely used zeolite catalyst. ZSM-5 catalysts are generally characterized by tetrahedral frameworks primarily composed of SiO2 with occasional aluminum substitutions. Because of their shape selectivity as molecular sieves and high degree of thermal stability, ZSM-5 catalysts are known for use in many hydrocarbon conversion reactions. It is advantageous at times to synthesize a catalyst so that its activity is directed to a specific hydrocarbon reaction. U.S. Pat. No. 4,968,650 discloses two methods for preparing ZSM-5 catalysts containing gallium primarily present in the framework of the catalyst and the use of such catalysts to produce high octane aromatics. The catalyst is disclosed for use in cracking and light paraffin upgrading, particularly converting C2-C12 paraffins, olefins and naphthenes to high octane aromatics.
Known processes for converting syngas to liquid hydrocarbons via methanol as an intermediate result in a broad distribution of hydrocarbons containing significant amounts of undesirable low carbon number hydrocarbons, e.g., C4 and lower and undesirable high carbon number hydrocarbons, e.g., C10 and higher. Production of such low carbon number hydrocarbons is highly inefficient as these hydrocarbons are not useful in gasoline products. Such high carbon number hydrocarbons require further processing such as cracking to form suitable hydrocarbons for use in gasoline. It would be desirable to have an improved process for conversion of syngas to liquid hydrocarbons via methanol with a minimum of processing resulting in higher yields of useful, high octane hydrocarbons boiling in the gasoline range. In view of current focus on stricter environmental standards, it would further be desirable for the improved process to produce reduced levels of aromatics as compared with known processes.